Edge-emitting etched-facet lasers

ABSTRACT

A laser chip having a substrate, an epitaxial structure on the substrate, the epitaxial structure including an active region and the active region generating light, a waveguide formed in the epitaxial structure extending in a first direction, the waveguide having a front etched facet and a back etched facet that define an edge-emitting laser, and a first recessed region formed in the epitaxial structure, the first recessed region being arranged at a distance from the waveguide and having an opening adjacent to the back etched facet, the first recessed region facilitating testing of an adjacent laser chip prior to singulation of the laser chip.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 14/595,848, filed Jan. 13, 2015, which is a continuation of U.S.patent application Ser. No. 13/690,792, filed Nov. 30, 2012, now U.S.Pat. No. 8,934,512, which claims the benefit of priority to U.S.Provisional Application No. 61/568,383, filed Dec. 8, 2011, and U.S.Provisional Application No. 61/619,190, filed Apr. 2, 2012, the entirecontents of each of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to etched-facet photonicdevices, and more particularly to improved etched-facet photonic devicesand methods for fabricating them.

Semiconductor lasers typically are fabricated on a wafer by growing anappropriate layered semiconductor material on a substrate throughMetalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy(MBE) to form an epitaxial structure having an active layer parallel tothe substrate surface. The wafer is then processed with a variety ofsemiconductor processing tools to produce a laser optical cavityincorporating the active layer and incorporating metallic contactsattached to the semiconductor material. Laser mirror facets typicallyare formed at the ends of the laser cavity by cleaving the semiconductormaterial along its crystalline structure to define edges, or ends, ofthe laser optical cavity so that when a bias voltage is applied acrossthe contacts, the resulting current flow through the active layer causesphotons to be emitted out of the faceted edges of the active layer in adirection perpendicular to the current flow. Since the semiconductormaterial is cleaved to form the laser facets, the locations andorientations of the facets are limited; furthermore, once the wafer hasbeen cleaved it typically is in small pieces so that conventionallithographical techniques cannot readily be used to further process thelasers.

The foregoing and other difficulties resulting from the use of cleavedfacets led to the development of a process for forming the mirror facetsof semiconductor lasers through etching. This process, as described inU.S. Pat. No. 4,851,368, also allows lasers to be monolithicallyintegrated with other photonic devices on the same substrate. It alsoallows wafer-level testing instead of cleaved bar testing that reducedcost of manufacturing. This work was further extended and a ridge laserprocess based on etched facets was disclosed in the IEEE Journal ofQuantum Electronics, volume 28, No. 5, pages 1227-1231, May 1992.However, FP and DFB edge-emitting lasers fabricated using etched facetscould not be closely packed because of the interference of the laseroutput from a front facet of laser to the back facet of the adjacentlaser that would distort on-wafer test results. The solution has been tospace out lasers, leaving wasted space between adjacent lasers. Forexample, for a DFB laser, this wasted space is on the order of 100 μmthat significantly reduces the number of useful laser chips that can beproduced from a given wafer.

SUMMARY OF THE INVENTION

In general, in one aspect, the invention features edge-emitting lasersthat are able to be fabricated with minimal wasted space betweenadjacent lasers. The front etched facet of a laser is facing a recessedregion on the back side of the adjacent laser chip so that the lightfrom the front facet of a first laser can exit without interference andback-reflection from the back facet of a second adjacent laser.

The invention may include a chip with a first etched-facet edge-emittingsemiconductor laser, the chip contains a recessed region to allow asecond etched-facet edge-emitting semiconductor laser, adjacent to thechip containing the first laser and with its front-facet facing thechip, to be closely spaced to each other while avoiding distortionduring the testing of the characteristics of the second laser. Thisallows a significant increase in the number of chips that can beproduced from a wafer.

Other embodiments include one or more of the following features. Therecessed region can have a slanted end wall to minimize back-reflection.The back facet of a laser may also face a recessed region on the frontside of the adjacent laser chip and the recessed region may have anangled end wall. Chips may have a complete recessed region or completeopening along their length and offset from each other to minimize backreflection.

In one particular embodiment, a laser chip may comprise a substrate; anepitaxial structure on the substrate, the epitaxial structure includingan active region, the active region generating light; a waveguide formedin the epitaxial structure extending in a first direction, the waveguidehaving a front etched facet and a back etched facet that define anedge-emitting laser; and a first recessed region formed in the epitaxialstructure, the first recessed region being arranged at a distance fromthe waveguide and having an opening adjacent to the back etched facet,the first recessed region facilitating testing of an adjacent laser chipprior to singulation of the laser chip.

In accordance with additional aspects of this particular embodiment, thefirst recessed region has a first end wall.

In accordance with additional aspects of this particular embodiment, thefirst end wall is at an angle other than normal to the first direction.

In accordance with additional aspects of this particular embodiment, theback etched facet is coated with a highly reflective material.

In accordance with additional aspects of this particular embodiment, thelaser chip may further comprise a second recessed region formed in theepitaxial structure and arranged at a second distance from the waveguidehaving an opening adjacent to the front etched facet, the secondrecessed region including a second end wall.

In accordance with additional aspects of this particular embodiment, thesecond end wall is at angle other than normal to the first direction.

In accordance with additional aspects of this particular embodiment, theopening to the first recessed region and the opening to the secondrecessed region are aligned to each other.

In another particular embodiment, the edge-emitting laser is a ridgelaser.

In accordance with additional aspects of this particular embodiment, theridge laser is of a Fabry-Perot (FP) type.

In accordance with additional aspects of this particular embodiment, theridge laser is of a distributed feedback (DFB) type.

In accordance with additional aspects of this particular embodiment, theedge-emitting laser is a Buried Heterostructure (BH) laser.

In accordance with additional aspects of this particular embodiment, theBH laser is of a Fabry-Perot (FP) type.

In accordance with additional aspects of this particular embodiment, theBH laser is of a distributed feedback (DFB) type.

In accordance with additional aspects of this particular embodiment, thesubstrate is InP.

In accordance with additional aspects of this particular embodiment, thesubstrate is GaAs.

In accordance with additional aspects of this particular embodiment, thesubstrate is GaN.

In another particular embodiment, a laser chip may comprise a substrate;an epitaxial structure on the substrate, the epitaxial structureincluding an active region, the active region generating light; a firstwaveguide formed in the epitaxial structure extending in a firstdirection, the first waveguide having a first front etched facet and afirst back etched facet that define a first edge-emitting laser; asecond waveguide formed in the epitaxial structure extending in thefirst direction, the second waveguide having a second front etched facetand a second back etched facet defining a second edge-emitting laser, arecessed region formed in the epitaxial structure, the recessed regionhaving an opening adjacent to one of the first back etched facet and thesecond back etched facet, the recessed region facilitating testing of anadjacent laser chip prior to singulation of the laser chip.

In accordance with additional aspects of this particular embodiment, atleast one of the first and second edge-emitting lasers is a ridgedistributed feedback (DFB) laser.

In accordance with additional aspects of this particular embodiment, atleast one of the first and second edge-emitting lasers is a BuriedHeterostructure (BH) distributed feedback (DFB) laser.

In another particular embodiment, a laser chip may comprise a substrate;an epitaxial structure on the substrate, the epitaxial structureincluding an active region, the active region generating light; awaveguide formed in the epitaxial structure extending in a firstdirection, the waveguide having a front etched facet and a back etchedfacet that define an edge-emitting laser; and a complete opening in theepitaxial structure in a direction parallel to and at a distance fromthe waveguide facilitating testing of an adjacent laser chip prior tosingulation of the laser chip.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages of the described embodiments will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing, and additional objects, features and advantages will beapparent to those of skill in the art from the following detaileddescription of preferred embodiments thereof, taken with theaccompanying drawings, in which:

FIG. 1 is a perspective view of edge-emitting etched-facet lasers.

FIG. 2 is a top plan view of two adjacent edge-emitting etched-facetlasers similar to FIG. 1.

FIG. 3(a) and FIG. 3(b) show the light vs. current and the spectralcharacteristics, respectively, of a Fabry-Perot edge-emittingetched-facet laser with relatively small separation between adjacentlasers.

FIG. 4(a) and FIG. 4(b) show the light vs. current and the spectralcharacteristics, respectively, of a Fabry-Perot edge-emittingetched-facet laser with relatively large separation between adjacentlasers.

FIG. 5 is a perspective view of edge-emitting etched-facet lasersaccording to the present invention.

FIG. 6 is a top plan view of two adjacent edge-emitting etched-facetlasers according to the present invention similar to FIG. 5.

FIG. 7 is a top plan view of adjacent edge-emitting dual-cavityetched-facet DFB lasers according to the present invention.

FIG. 8 is a top plan view of edge-emitting etched-facet lasers withlaser chips offset from each other in each row of lasers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a perspective view of a substrate 90 with an epitaxiallydeposited waveguide structure including an active region 80 in whichetched-facet lasers including a ridge 40 are fabricated. Theetched-facet ridge laser mesas 10 and 20 are positioned on the substrateso that the front facet 50 of laser corresponding to mesa 10 is at adistance 70 from the back facet of laser corresponding to mesa 20.

FIG. 2 shows a top plan view of two adjacent etched-facet ridge lasers,similar to those in FIG. 1. A wire-bond pad 30 is provided to allowwire-bonding to the pad and for electric current to be directed to theridge allowing the laser to operate. The laser mesas 10 and 20 arepositioned on chip 15 and 25, respectively. The chips 15 and 25 areformed through a singulation process along the lines that define theboundary of the chips in FIG. 2.

The substrate 90 may be formed, for example, of a type III-V compound,or an alloy thereof, which may be suitably doped. The substrate, such asInP, includes a top surface on which is deposited, as by an epitaxialdeposition such as Metalorganic Chemical Vapor Deposition (MOCVD) orMolecular Beam Epitaxy (MBE), a succession of layers which form anoptical waveguide that includes an active region 80. The semiconductorlaser structure may contain upper and lower cladding regions, formedfrom lower index semiconductor material than the active region 80, suchas InP, adjacent the active region 80, which may be formed withInAlGaAs-based quantum wells and barriers. The lower cladding may beformed partially through the epitaxial deposition and partly by usingthe substrate. For example, a 1310 nm emitting epitaxial structure canbe used with the following layers on an InP substrate 90: 0.5 μm n-InP;0.105 μm AlGaInAs lower graded region; an active region 80 containingfive 6 nm thick compressively strained AlGaInAs quantum wells, eachsandwiched by 10 nm tensile strained AlGaInAs barriers; 0.105 μmAlGaInAs upper graded region; 1.65 μm thick p-InP upper cladding; andhighly p-doped InGaAs cap layer. The structure may also have a wet etchstop layer.

One of the key benefits of etched-facet lasers is that testing isperformed at wafer-level as opposed to bar-level testing forcleaved-facet lasers. However, to allow the full benefit of on-wafertesting, the distance 70 should be large enough to prevent the adverseimpact of back-reflection and interference from back-facet 60 to frontfacet 50. The front facet 50 is where most of the light emerges from theedge-emitting laser corresponding to mesa 10. For example, if thedistance 70 is 50 μm for an InP-based 1310 nm Fabry-Perot (FP) ridgelaser of ridge width of about 2 μm, the undesirable characteristics dueto the back-reflection are observed in FIG. 3, where FIG. 3(a) shows thelaser Light vs. Current (or LI) characteristics starts to change ataround 50 mA and a corresponding problem shows up in FIG. 3(b) above 50mA: the spectral characteristics of the laser have an abnormal doubledistribution of FP modes at currents above 50 mA. Each spectrum isoffset in FIG. 3(b) for clarity.

By increasing the distance 70 to 100 μm or more for the 2 μm ridge width1310 nm FP lasers, the impact from the back-reflection and interferenceis minimized and the adverse impact is no longer seen in the LIcharacteristics of FIG. 4(a) and the spectrum of FIG. 4(b), where eachspectrum is offset for clarity. The spectral results in FIG. 4(b) show anormal distribution for the FP modes. The distance 70 is also veryimportant in on-wafer-testing of distributed feedback (DFB) lasers,wherein a grating is incorporated in the epitaxial structure. If thedistance 70 is too small, back-reflection can cause a change in thecharacteristics of the DFB laser and as a result the lasercharacteristics obtained after singulation may be different,significantly reducing the value of on-wafer testing. By choosing adistance 70 of 100 μm or more, the undesirable impact of theback-reflection has been eliminated for 1310 nm DFB lasers. However,increasing the distance 70 to 100 μm or more is the wasted space thatsignificantly reduces the number of chips that can be obtained from awafer.

FIG. 5 shows a perspective view of a substrate 190 with an epitaxiallydeposited structure, similar to the one described above, including anactive region 180 in which etched-facet lasers including a ridge 140 arefabricated. The etched-facet lasers are arranged on the substrate in analternating fashion front-to-back and back-to-front so that the planesof the front and back facets of two neighboring laser mesas face eachother (and correspondingly, the planes of the back and front facets oftwo neighboring laser mesas face each other). In this arrangement, theetched ridge 140 of laser mesa 110 is aligned with recess 157 that isformed in neighboring laser mesa 120. The etched-facet ridge laser mesas110 and 120 are positioned on the substrate so that the front facet 150of laser corresponding to mesa 110 is at a distance 170 from the wall155 of the recessed region 157. The wall 155 can be at an angle otherthan normal to the incident laser beam to minimize back-reflection tothe front facet 150. The recessed region is around 5 μm deep, so thatthe wall 155 is around 5 μm in height and the floor of the recessedregion is about 2.9 μm below the plane of the active region. In the casewhere the wall 155 is at an angle other than the normal, the distance170 can be reduced down from 100 μm, while avoiding the back-reflectionand interference at levels that are detrimental to on-wafer testing.

FIG. 6 illustrates a top plan view of two adjacent edge-emittingetched-facet lasers, similar to FIG. 5. In practice, the two adjacentedge-emitting lasers of FIG. 6 are used as a unit building block andplaced on the wafer in rows and columns. The back facet 160 of lasercorresponding to mesa 120 is high reflectivity coated, and as such, theimpact of back-reflection is not too great. Nevertheless, a recessedregion 167 can be provided for minimizing back-reflection. The wall 165of the recessed region 167 can also be at an angle other than normal tothe incident laser beam to minimize back-reflection to the back facet160. The laser mesas 110 and 120 are positioned on chip 115 and 125,respectively. The chips 115 and 125 are formed through a singulationprocess along the lines that define the boundary of the chips in FIG. 6.The chip 115 including a wire-bond pad 130. The separation 175 betweenlaser mesas 110 and 120 is just large enough (around 10 μm) to allow thesingulation to occur. The minimization of separation 175 has allowed asignificantly larger number of laser chips to be produced from the samesize wafer. Further, the recessed regions 157 or 167 may be formed atthe same time that the dry etched facet and dry etched ridge are formed.

FIG. 7 shows the application of the present invention to the case of twolaser cavities per chip. The laser mesas 210 and 220 are positioned onchip 215 and 225, respectively. The chips 215 and 225 are formed througha singulation process along the lines that define the boundary of thechips in FIG. 7. The chip 215 comprises two laser ridge cavities 240 and241 that have wire-bond pads 230 and 231, respectively, providingelectrical current to the ridges 240 and 241. Ridge laser 240 has afront facet 250 and ridge laser 241 has a front facet 251. The adjacentmesa has a recessed region 257 in front of facets 250 and 251. Therecessed region has a termination of two walls 255 and 256 that can beat an angle off from the normal to each laser beams emerging from facets250 and 251, respectively. The distance 270 between the front facet 250and the wall 255 is preferably more than 100 μm, but could be shorter ifthe wall is at an angle to the normal of the laser beam emerging fromfacet 250. Similarly, the distance 271 between the front facet 251 andthe wall 256 is preferably more than 100 μm, but could be shorter if thewall is at an angle to the normal of the laser beam emerging from facet251. This allows the laser mesas 210 and 220 to have a minimalseparation 275 to increase the number of chips that can be produced froma given wafer. The back facets 260 and 261 of the two lasers in mesa 220can also face a recessed region 267 in the mesa 210 to minimizeback-reflection.

Another alternative according to the present invention is illustrated inFIG. 8, where the etched-facet edge-emitting laser mesas 310, 320, 321and 322 are positioned on chips 315, 325, 326 and 327, respectively, andthe lasers have a ridge 340 and a wire-bond pad 330 to provideelectrical current to the ridge, as illustrated for laser correspondingto mesa 310. The recessed region 357 is formed completely on one side ofthe etched-facet laser chip 320 extending the full length of the chip,forming a complete opening. In this way, the recessed region 357 extendsto the back facet 355 of a laser corresponding to mesa 321. The backfacet 355 is usually normal to the emitted laser light that emerges fromfront facet 350. The distance 370 is the distance between front facet350 and back facet 355. Laser mesas 310 and 320 have minimal separation375 in the top-to-bottom direction, similar to that in FIGS. 6 and 7;however, laser mesas 321 and 322 also have minimal separation 376 in theleft-to-right direction. This further allows an increase in the numberof chips that can be produced from a given wafer. The sides of chips areonly aligned in left-to-right, as opposed to the chips in FIGS. 6 and 7that are aligned in both left-to-right and top-to-bottom. Thesingulation needs to accommodate this fact, and for example, thesingulation needs to occur in the left-to-right direction first, andthen chips separated by top-to-bottom singulation. Although FIG. 8 andthe above description are in terms of a single laser cavity per chip, itwill be understood that the same approach would be applicable to two ormore laser cavities per chips.

The current invention is described using a 1310 nm emitting laser thatis based on an InP substrate. However, a number of other differentepitaxial structures based on InP, GaAs, and GaN substrates, forexample, can benefit from this invention. Numerous examples of epitaxialstructures including active layers on these exemplary substrates areavailable that emit, for example, wavelengths in the infrared andvisible regions of the spectrum. Further, although an edge-emittingridge laser having an etched ridge has been described, it will beunderstood that other types of etched-facet lasers, such as etched-facetBuried Heterostructure (BH) lasers, can be used.

Although the present invention has been illustrated in terms of variousembodiments, it will be understood that variations and modifications maybe made without departing from the true spirit and scope thereof as setout in the following claims. Further, it will be understood that thedimensions and proportions shown in the figures are not necessarily toscale, but are used to clearly illustrate the salient features of thestructure and method.

1. A method of fabricating a semiconductor laser structure comprising:epitaxially depositing a structure on a substrate, the structureincluding an active region for generating light; forming first andsecond waveguides in first and second portions of the structure,respectively, the first waveguide having a first front etched fact and afirst back etched facet, the second waveguide having a second frontetched fact and a second back etched facet; forming a recessed region inthe first portion of the structure, the recessed region having anopening directly opposing the second front etched facet of the secondwaveguide, the recessed region facilitating testing of the secondwaveguide prior to singulation of the second portion of the structurefrom the substrate.
 2. The method of claim 1, wherein the recessedregion comprises an end wall upon which light from the second frontetched facet of the second waveguide impinges.
 3. The method of claim 2,wherein the first waveguide extends in a first direction, wherein theend wall is at an angle other than normal to the first direction toreflect light from the second front etched facet impinging upon the endwall.
 4. The method of claim 1, further comprising coating at least oneof the first back etched facet and the second back etched facet with ahighly reflective material.
 5. The method of claim 1, wherein the firstfront etched fact and the first back etched facet of the first waveguidedefine a first edge-emitting laser.
 6. The method of claim 5, whereinthe edge-emitting laser is a ridge laser.
 7. The method of claim 6,wherein the ridge laser is of a Fabry-Perot (FP) type.
 8. The method ofclaim 6, wherein the ridge laser is of a distributed feedback (DFB)type.
 9. The method of claim 5, wherein the edge-emitting laser is aBuried Heterostructure (BH) laser.
 10. The method of claim 9, whereinthe BH laser is of a Fabry-Perot (FP) type.
 11. The method of claim 9,wherein the BH laser is of a distributed feedback (DFB) type.
 12. Themethod of claim 1, wherein the recessed region is a first recessedregion, further comprising: forming a second recessed region in thefirst portion of the structure, the second recessed region having anopening directly opposing a third back etched facet of a third waveguidein a third portion of the structure, the second recessed regionfacilitating coating of the third back etched facet with a highlyreflective material prior to singulation of the third portion of thestructure from the substrate.
 13. The method of claim 12, wherein theopening of the first recessed region and the opening of the secondrecessed region are aligned to each other in the first portion of thestructure.
 14. The method of claim 1, wherein the recessed region is afirst recessed region, further comprising: forming a second recessedregion in the second portion of the structure, the second recessedregion having an opening directly opposing a first back etched facet ofthe first waveguide, the second recessed region facilitating coating ofthe first back etched facet with a highly reflective material prior tosingulation of the second portion of the structure from the substrate.15. The method of claim 14, further comprising: forming a third recessedregion in the second portion of the structure, the third recessed regionhaving an opening directly opposing a third front etched facet of athird waveguide in a third portion of the structure, the third recessedregion facilitating testing of the third waveguide prior to singulationof the third portion of the structure from the substrate.
 16. The methodof claim 15, wherein the opening of the second recessed region and theopening of the third recessed region are aligned to each other in thesecond portion of the structure.
 17. The method of claim 15, wherein thesecond waveguide extends in a second direction, wherein the secondrecessed region comprises a second end wall upon which light from thethird front etched facet of the third waveguide impinges.
 18. The methodof claim 17, wherein the second end wall is at an angle other thannormal to the second direction to reflect light from the third frontetched facet impinging upon the second end wall.
 19. The method of claim1, wherein the substrate is formed of one of: InP, GaAs, and GaN. 20.The method of claim 1, wherein the first portion of the structure is acommon contiguous portion of the structure comprising the recessedregion and the first front and back etched facets defining a firstedge-emitting laser, wherein the first portion of the structure isspaced apart from and does not include the second portion of thestructure.